U.S. patent number 5,334,903 [Application Number 07/985,988] was granted by the patent office on 1994-08-02 for composite piezoelectrics utilizing a negative poisson ratio polymer.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to Wallace A. Smith.
United States Patent |
5,334,903 |
Smith |
August 2, 1994 |
Composite piezoelectrics utilizing a negative Poisson ratio
polymer
Abstract
A piezoelectric composite which can be used as a transducer is
constructed of parallel piezoelectric ceramic rods set in a passive
polymer matrix which has a negative Poisson ratio whereby the
electromechanical coupling of the transducer is greatly
increased.
Inventors: |
Smith; Wallace A. (Vienna,
VA) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
|
Family
ID: |
25531979 |
Appl.
No.: |
07/985,988 |
Filed: |
December 4, 1992 |
Current U.S.
Class: |
310/358;
310/800 |
Current CPC
Class: |
H04R
17/005 (20130101); H01L 41/183 (20130101); Y10S
310/80 (20130101) |
Current International
Class: |
H01L
41/18 (20060101); H04R 17/00 (20060101); H01L
041/187 (); H04R 017/00 () |
Field of
Search: |
;310/358,359,800,311
;367/157 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Transverse Honeycomb Composite Transducers" by A. Safari et al.
Nov. 13, 82 Penn. State Univ. .
T. R. Gururaja, et al. "Piezoelectric Ceramic-Polymer Composites
For Transducer Applications" Electric Ceramics, pp. 92-128, 1987.
.
K. E. Evans, "Tensile Network Microstructures Exhibiting Negative
Poisson's Ratio," J. Phys. D:Appl. Phys. 22, 1989. .
Ken Evans, "Tailoring the Negative Poisson Ratio", Chemistry &
Industry, Oct. 1990. .
Wallace A. Smith, The Role of Piezocomposites in Ultrasonic
Transducers, Proceedings of IEEE, 1989..
|
Primary Examiner: Dougherty; Thomas M.
Attorney, Agent or Firm: McCarthy; William F. McDonald;
Thomas E. Busch; James T.
Claims
What is claimed is:
1. A piezoelectric ceramic-polymer composite material comprising: a
matrix of a polymer which has a negative Poisson ratio, a plurality
of piezoelectric ceramic rods embedded in said matrix each of which
rods has its longitudinal axis aligned in the same direction; and
means for polarizing the composite with an electric field parallel
to the axis of said rods.
2. The composite of claim 1 in which the connectivity between
materials is 1-3.
3. The composite of claim 2 in which the piezoelectric ceramic is a
niobium doped PZT material.
4. The composite of claim 3 in which the said polymer is an
epoxy.
5. The composite of claim 4 further including electrodes connected
on opposite sides of said composite so that the composite can be
used as a transducer for a hydrophone.
6. The composite of claim 4 further including electrodes connected
on opposite sides of said composite so that the composite can be
used as an ultrasonic transducer.
7. A piezoelectric ceramic-polymer composite material of 1-3
connectivity for use as a transducer comprising: a passive polymer
matrix which exhibits a negative Poisson ratio, a plurality of PZT
piezoelectric rods embedded in said matrix, each of said rods
having its longitudinal axis aligned in the same direction; and
means for polarizing the composite with an electric field parallel
to the axis of said rods.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to improved composite piezoelectric
materials which find use in ultrasonic applications such as
transducers used in naval sonar and in medical ultrasonic imagers.
Piezoelectric materials are also used to achieve hydrostatic
electromechanical coupling which characteristic can be used in
passive hydrophones. In addition, numerous other applications of
piezoelectric materials have been developed as electromechanical
and electroacoustic transducers.
Although single crystal piezoelectric materials retain their
utility and dominate certain special arenas, such as frequency
stabilized oscillators, in applications ranging from watches to
radar, and surface acoustic wave devices, in applications ranging
from television filters to analogue signal correlators, new
piezoelectric materials combining a piezoelectric ceramic with a
passive polymer have now come to the forefront of the market. Our
invention relates to these newer piezoceramic composites and their
numerous uses. In particular, our invention contemplates the use of
negative Poisson's ratio materials as the passive polymer in the
composite structure.
The key consideration in the design; whether for a sensor, an
actuator, or both simultaneously; is to ensure the maximum
efficiency in electromechanical energy conversion. There are many
engineering applications' demands--weight, flexibility,
environmental stability, electrical impedance matching, acoustic
radiation coupling, cost--that will pull the design away from this
optimum, but it is best to aim first at the right target. There are
usually many ways to compensate for the piezomaterial's shortfalls
in meeting design criteria other than electromechanical energy
conversion: acoustic matching layers, electrical transformers,
environmental coatings, buoyant ballast. But the piezoelectric
material is the only place where electromechanical energy
transformation is accomplished. Maximizing the composite's
electromechanical coupling should be the first aim in the
piezocomposite material design.
In pulse-echo applications, a rod composite piezoelectric is more
effective at electromechanical energy conversion than is its
constituent piezoceramic. While this seems counter-intuitive at
first blush, a quick glance at FIG. 2 clarifies the issue. That
figure depicts in cross-section the response of a thin composite
plate to a high frequency pressure wave impinging on its faces. At
the high frequencies employed in pulse-echo applications, the thin
plate is so wide that it is inertially clamped as a whole in the
lateral directions. That is, the pressure on the faces reverses so
fast that the plate does not have time to bulge or contract in the
sideways direction. This lateral clamping reduces the total
displacement and total piezoelectric charge produced in a solid
plate of piezoceramic. In the piezocomposite structure, however,
the thin ceramic rods are free to expand or contract in the
sideways direction at the expense of the much softer polymer which
surrounds them. A piezocomposite plate can have a thickness-mode
electromechanical coupling constant (.about.60-70%), much larger
than the thickness-mode coupling of a solid ceramic plate
(.about.45-50%), approaching even the coupling of a long ceramic
rod (.about.70-75%). FIG. 3 shows how the thickness-mode coupling
constant for a composite plate varies with volume fraction of
piezoceramic for three different polymers. The composite's
thickness mode coupling exceeds that of the component ceramic for
all but the lowest volume fractions; moreover, a softer polymer
permits higher values.
In hydrostatic applications, the effectiveness of a piezoelectric
material for electromechanical energy-conversion is measured by the
hydrostatic coupling coefficient,
where d.sub.h is the material's hydrostatic current responsivity,
.epsilon..sub.33.sup.T its dielectric permittivity, and
s.sub.h.sup.E the material's hydrostatic compliance.
The hydrostatic current responsivity, d.sub.h =d.sub.33 +2d.sub.31,
has two contributions--one from pressure on the faces of the plate,
d.sub.33, and the other from pressure on the sides, d.sub.31. These
two contributions are typically of opposite sign and nearly equal
in magnitude. In the piezocomposites, we can increase d.sub.h by
reducing the negative lateral contributions.
FIG. 4A depicts the contribution to the hydrostatic current
responsivity of the composite from the force exerted on the faces
of the plate. The effect is similar to the uniaxial response shown
in FIG. 2 above except that, at low frequencies, the entire plate
is free to expand laterally. The essential role of the polymer is
to transfer the force falling on it to the adjacent ceramic. The
d.sub.33 of the composite is nearly equal to the d.sub.33 of the
constituent piezoceramic since nearly all the force falling on the
plate is transferred to the ceramic.
A top view of the composite plate is shown in FIG. 4B which
portrays a portion of the contribution to the hydrostatic current
responsivity from the force exerted on the sides of the composite
plate. Here the polymer transfers part of the lateral force to the
ceramic and bears part of the lateral force itself in the regions
between the piezoceramic rods. To reduce these negative
contributions to the hydrostatic response, we can add reinforcing
fibers to these polymer paths so that more of the lateral force is
borne by the piezoelectrically passive part of the structure.
A side view of the composite plate in FIG. 4C which shows the
remaining portion of the contribution to the hydrostatic current
responsivity from the force exerted on the sides of the composite
plate. Here the polymer both presses on the sides of the ceramic
rods as well as bulges up. The direct pressure on the sides of the
ceramic was accounted for above. The bulging of the polymer,
however, is a new effect. This bulging causes the polymer to pull
on the ceramic rods trying to lengthen them, thereby producing a
contribution to the d.sub.31 of the composite from the d.sub.33 of
the ceramic. This Poisson-ratio effect is an important contribution
to the composite's d.sub.31. To minimize this contribution to the
composite's d.sub.31 we can reduce the polymer's Poisson ratio by
adding air bubbles. Foaming the polymer is an effective strategy
for reducing these deleterious contributions to the hydrostatic
response, but unfortunately introduces an unwanted pressure
dependence because these air pockets can collapse under high static
bias pressures.
FIG. 5A shows that the d.sub.33 coefficient rises monotonically
with ceramic fraction: as the amount of ceramic increases, more of
the force on the faces of the plate is borne by the
piezoelectrically active ceramic and less by the passive polymer.
Since ceramics are typically an order of magnitude stiffer than the
polymer, the composite's d.sub.33 attains almost the value of the
ceramic's d.sub.33 already at low ceramic fraction. At low ceramic
content, the more stiff the ceramic, the greater fraction of the
force on the ceramic. At moderate ceramic content, the shifting of
the load from the polymer to the ceramic saturates and little more
remains to be gained since once the ceramic is carrying most of the
external force, it can do no more.
The curves of FIG. 5B show the variation of the composite's
d.sub.31 coefficient in the solid curve, as well as, in the dotted
curves, the two contributions that sum up to it, namely, d.sub.31
=.alpha.d.sub.31 +.beta.d.sub.33, where the .alpha.d.sub.31 term is
the contribution from the ceramic's d.sub.31, and .beta.d.sub.33
term contains the contribution from the ceramic's d.sub.33. The
enhancement of the composite's d.sub.31 by the .beta.d.sub.33 term
is deleterious. It stems from the stress on the composite's sides
causing the more compliant polymer to bulge vertically more than
the ceramic; the bulging polymer acts to extend the rods, producing
a charge directly from the ceramic's d.sub.33.
The curve in FIG. 5C shows the composite's hydrostatic charge
response, d.sub.h =d.sub.33 +2d.sub.33, which is just the sum of
the two previously described curves. The composite structure
enhances d.sub.h over the ceramic's d.sub.h at low volume
fraction.
While the piezoelectric coefficient--discussed above--is central to
the use of piezocomposites as hydrostatic transducer materials,
other properties of the material are important in determining their
suitability for devices, in particular their dielectric and elastic
coefficients. FIG. 6 shows the variation with ceramic fraction of
the dielectric permittivity, .epsilon..sub.33.sup.T, and the
hydrostatic compliance, S.sub.h.sup.E. Both of these properties
interpolate nearly linearly between the values for the pure polymer
and pure ceramic. In the case of the dielectric constant, the
slight suppression at low ceramic fraction is due to partial
mechanical clamping of the ceramic by the softer--but greater in
amount--polymer. In the elastic arena, a modest curvature occurs at
low ceramic fraction; this stems from the effectiveness of small
amounts of ceramic in stiffening the composite plate in the
direction along the rods. Combining the previously calculated
hydrostatic charge coefficient, dhd h, with these elastic and
dielectric responses yields the hydrostatic electromechanical
coupling constant which reveals the advantage of the composite
structure, namely, the composite's k.sub.h exceeds the ceramic's
k.sub.h.
Description of the Related Art
Designing new piezoelectric materials by combining a piezoelectric
ceramic with a passive polymer has been a research theme now for
more than a decade. The publication of R. R. Gururaja et al is a
thorough review of the state of the art of ceramic polymer
composites up to 1987. U.S. Pat. No. 4,728,845 to HAUN et al
discloses the use of a piezoelectric composite consisting of lead
zirconate-titanate (PZT) and a polymer for use in hydrophones. HAUN
enhances the piezoelectric charge and voltage coefficients by using
a void to isolate PZT rods from the polymer.
The publication of Smith Role of Piezocomposites in Ultrasonic
Transducers, Proceeding of IEEE, 1989 discloses the use of
piezoceramic and passive polymers in ultrasonic medical imagers.
The U.S. Pat. No. 4,613,784 to HAUN et al also is concerned with
enhancing the hydrostatic piezoelectric voltage coefficient of a
PZT-polymer composite. In this patent, transversly reinforced glass
fibers are used to achieve the enhanced coefficient. The first
publication of K. E. Evans "Tensile Network Microstructures
Exhibiting Negative Poisson's Ratio", J. Phys. D:APPL./PHYS. 22
1989.
On page 1875 of this publication Evans discloses that his
previously described tensile microstructures can be found in
polymers details various microstructures which can exhibit a
negative Poisson ratio. The second K. E. Evans "Tailoring the
Negative Poisson Ratio", Chemistry and Industry, Oct. 1990. In this
reference on page 656, polytetrafluorothylene (PTFE) is disclosed
as exhibiting a large negative Poisson ratio. On the same page,
reference is made to ultra-high molecular weight polyethylene and
other polymers publication explores the tailoring of the negative
Poisson ratio material. Our invention utilizes a negative Poisson
ratio material for its improved performance characteristics. The
U.S. Pat. No. 4,668,557 to Roderic Lakes describes a method of
making negative Poisson Ratio materials.
SUMMARY OF THE INVENTION
This invention is intended to maximize the electromechanical
coupling in 1-3 piezocomposites by using materials for the passive
polymer phase that possesses negative Poisson's ratio. The negative
Poisson's ratio materials are used to redirect external stress to
bear on the piezoceramic material. An object of the invention is to
enhance substantially the electromechanical coupling coefficient
for both thickness-mode and hydrostatic mode in 1-3 composite
piezoelectric materials. Using the improved materials in pulse-echo
transducers used in resonant thickness-mode will lead to increased
transducer bandwidth, lower insertion loss, and more compact
impulse response--all critical measures of the transducer's
performance and utility. Using the improved materials in hydrophone
sensors or hydrostatic actuators used in non-resonant
thickness-mode will lead to increased sensitivity as a receiver and
increased efficiency as an actuator--critical performance
characteristics for naval sonal transducers. Moreover, these
improved hydrostatic performance characteristics will be stable
under hydrostatic bias pressure encountered by naval transducers
opiating at great depths in the ocean.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the rod composite or 1-3 composite-geometry that
is contemplated for the invention.
FIG. 2(a) is a schematic side view of a composite plate being
squeezed on its faces by a high frequency sound wave.
FIG. 2(b) is a schematic side view of the undisturbed plate.
FIG. 3 is a variation of the thickness mode electromechanical
coupling constant of a piezocomposite plate with ceramic volume
fraction, as predicted from a simple physical model, for three
different polymers: stiff (solid), firm (dashed), and soft
(dotted).
FIGS. 4(a), 4(b), and 4(c) are schematic representations of the
physical effects behind the composite piezoelectric charge
coefficients.
FIG. 5(a), 5(b), and 5(c) show the piezoelectric charge
coefficients for a composite made from conventional PZT 5 and
Stycast, a firm polymer.
FIGS. 6(a), 6(b), 6(c), and 6(d) show the variation of dielectric
and elastic coefficient with the volume fraction of ceramic in
percent.
FIG. 7 depicts the hydrostatic coupling factor versus the volume
fraction of piezoelectric ceramic for various Poisson's ratios.
FIG. 8 shows the calculated values of K.sub.t versus ceramic
fraction for three different Poisson Ratios.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a piece 10 of composite PZT-polymer is shown
which consists of ceramic rods 11 imbedded in a polymer matrix 12.
Orthogonal axes X.sub.1, X.sub.2 and X.sub.3 are shown as a
reference for direction and to aid in describing a connectivity
pattern. In our notation of 1-3, for example, one (1) refers to the
one dimensionally connected PZT phase and three (3) refers to the
three dimensionally connected polymer phase. Connected to the
composite piece are electrodes 13 and 14.
Although our preferred embodiment utilizes PZT, it should be
understood that the principle of the invention could utilize any
piezoelectric ceramic material, i.e., PZT5 which is
Pb(Zi,Ti)O.sub.3. Similarly, the polymer could be one of many
materials, i.e., PFE or polyurethane as discussed in the Evans
publications "Tailring the Negative Poisson Ratio" or an epoxy as
disclosed in the Haun, et al. patent.
The most widely used method to make 1-3 piezocomposites is the
dice-and-fill technique. With this method, two sets of deep grooves
are cut in a block of piezoceramic at right angles to each other, a
polymer is cast into these grooves, and the solid ceramic base is
ground off. After polishing the plate to final thickness,
electrodes are applied to the faces, and the ceramic is poled by
applying a strong electric field, usually at slightly elevated
temperatures. For high-frequency operation, fine spatial scales are
required; this presents severe demands on the machinability of the
ceramic as well as on the machining technology itself. Kerfs of 25
microns and below are achievable, using diamond impregnated dicing
wheels on OD saws developed for the semiconductor industry to dice
chips from a processed silicon wafer. A fine-grained, high-density
piezoceramic is essential if the pillars are to survive this
machining.
The above dice-and-fill technique can be varied so that the need
for polishing to final thickness is eliminated. This variation
proceeds in two steps: first, in a solid ceramic plate of desired
thickness, two sets of grooves at right angles to each other are
cut halfway through, and a polymer is vacuum cast into the grooves
with a lid waxed onto the top of the ceramic to prevent any polymer
from coating the tops of the pillars. Second, the plate is turned
over and the process is repeated with the new grooves aligned with
the already filled grooves in the lower half. After the second
polymer fill, the capping lids are removed, the plate is electroded
and poled. The final composite thickness is set by the initial
thickness of the solid ceramic plate; no polishing is necessary.
Grooves only half the desired composite thickness are cut; finer
scales can be made with less risk of pillar fracture.
A third method of making our invention which can be used if round
bars are desired is described in the HAUN, et al U.S. Pat. No.
4,728,845, Col 2, line 62 to Col 3, line 2 which is hereby
incorporated by reference.
The invention maximizes the electromechanical coupling in the 1--3
piezocomposites by using materials for the passive polymer phase
that possess negative Poisson's ratio. To understand how this is
achieved, it is useful to review the status of research on
materials with negative Poisson's ratio.
If we press on the top and bottom of a piece of material, it not
only gets shorter, but also, typically, bulges out to its sides.
This lateral bulging is known as the Poisson effect, and is
characterized by the Poisson ratio, defined as minus the lateral
strain divided by the longitudinal strain. The minus sign is
introduced so that positive values are obtained; this minus sign
incorporates our normal experience that when you squeeze something
it contracts along the direction where pressure is applied but
expands in the perpendicular directions. In terms of the elastic
compliance, s.sub.ij, of an isotropic material, Poisson's ratio,
.sigma., is just -s.sub.12 /s.sub.11. This ratio cannot take on
just any value. If the solid is in a stable equilibrium state, the
elastic compliance must be a positive definite matrix; this
constrains the Poisson's ratio of an isotropic medium to lie
between -1 and +1/2. There is no physical requirement that
Poisson's ratio be positive. The fact that most materials have
positive Poisson's ratio,typically near 0.3,is an accidental
occurrence, not a logical necessity.
Materials exhibiting negative Poisson's ratio occur naturally, but
only rarely. The recent swell of interest in these materials stems
from the fact that they can be crafted by tailoring a material's
microstructure. To make a material of this type, a polymer foam is
isotropically crushed by a substantial amount ,i.e., factor of two
or three in volume and then annealed to retain the deformed state
as its new equilibrium. This new isotropic material with a
reentrant foam microstructure has the desired property of a
negative Poisson's ratio. A more complete discussion of a method of
making negative Poisson Ratio materials is described in the U.S.
Pat. No. 4,668,557 issued to Roderic Lakes which is herein
incorporated by reference in this application.
The projected improvements in piezocomposite performance can be
realized in a variety of ways. A negative Poisson's ratio material
might be synthesized separately, ground up, and inserted as a
filler in a normal polymer used in the conventional dice-and-fill
fabrication method. Alternately, the material might be made in
ribbon or fiber form, interspersed with piezoelectric fibers, and
finally fused into a bundle which can be sliced into the desired
plate form. The most effective embodiment of this idea requires
negative Poisson's ratio materials with high elastic modulus.
A critical restriction is that the lateral rod spacing in the
composite be sufficiently fine that any direct vertical stress
impinging on the polymer be effectively transferred laterally to
the piezoceramic rod. If this is the case, the piezocomposite plate
oscillates uniformly across its face and the composite acts as an
effective homogeneous medium. This is a restriction on material
performance. If the spacings are too coarse, the acoustic energy
incident on the polymer portion of the transducer is lost and
transducer performance suffers--even if we could devise a simple
and effective model for its performance. The safe range of spacings
depends, in first order, on the shear modulus of the polymer phase.
If the polymer's shear modulus is high, stresses pressing on the
polymer in the face of the transducer are readily transferred
laterally to the ceramic rods which convert that acoustic energy
into electrical form; then, relatively wide spacing of the rods is
permitted. Conversely if the polymer is soft under shear, the rods
must be closely spaced, placing substantial demands--both in
technologky and in cost--on the material fabrication.
In 1-3 piezocomposite plates being optimized for the thickness-mode
resonance used in pulse-echo imaging transducers, the polymer phase
plays another important role: being softer than the piezoceramic,
the polymer allows the rods to laterally bulge or contract while
the composite plate as a whole remains inertially clamped in the
lateral direction. This means that the external stresses tap into
the larger (typically 70-75%) rod--or laterally
free--electromechanical coupling factor, k.sub.33, rather than the
small (typically 45-50%) plate--or laterally clamped--coupling
factor of the piezoceramic. Of course, a normal polymer will always
provide some partial lateral clamping, so the maximum
electromechanical coupling of the ceramic's k.sub.33 is not
attained by the composite plate.
The newly devised polymers with negative Poisson's ratio enable us
to lessen or even totally escape this constraint. When a sound wave
presses on the top of a composite plate containing a polymer with
negative Poisson's ratio, the polymer will shorten and pull in
laterally. This lateral contraction of the polymer not only lets
the ceramic expand freely but also--if we could design the right
polymer--pulls out on the sides of the ceramic rods. This way we
might achieve the stress pattern on the ceramic that provides its
maximal coupling constant k.sub.i3. FIG. 8 plots the calculated
values of k.sub.t versus ceramic fraction for selected values of
the Poisson's ratio ranging from a typical normal value of +0.3 to
the opposite of -0.3. The negative Poisson's ratio provides clear
advantages.
Another benefit is provided by a polymer with negative Poisson's
ratio: its shear modulus, c.sub.44 =[2s.sub.11 (1+.sigma.)].sup.-1,
is larger. Thus, for a polymer with a given compliance, s.sub.11,
the negative .sigma. would permit a wider spacing of rods,
lessening demands on material fabrication. Not all problems are
solved however, Even with a negative Poisson's ratio, the polymer
will still have a finite elastic modulus and will consume energy to
compress. This effect dominates, of course, at low volume fractions
of ceramic, as we see in FIG. 8.
Designing a 1-3 piezocomposite plate to respond to a hydrostatic
stress is a daunting task because the hydrostatic stress pattern
has very nearly zero coupling in the best piezoceramic, lead
zirconate-titanate. Indeed, modified lead titanates are often
preferred in this application in spite of their considerably lower
maximal coupling coefficient k.sub.i3 (50-55% versus 70-75% in PZT)
and dielectric constant (200-500 versus 1000-6500 in PZT).
In this application, not only is the incident isotropic planar
stress transmitted directly to the rods, but also the Poisson
effect in the polymer phase converts the planar stress to a
vertical stress which opposes the incident vertical stress. This,
of course, occurs with a normal polymer with positive Poisson's
ratio. So, the new negative Poisson's ratio materials have a useful
role to play here too. By converting a planar compressive stress
into a vertical compressive stress, such new polymers redirect
energy to reinforce the vertical compressive stress not oppose it.
FIG. 7 shows the enhancements in hydrostatic coupling constant that
can be achieved using polymers with negative Poisson's ratio.
The need to suppress the polymer's Poisson effect for the
hydrophone application has been understood for some time. The two
HAUN, et al patents disclose two methods of accomplishing this by
introducing a void into the polymer to reduce .sigma.; this
introduces an undesired bias pressure dependence to the hydrophone
sensitivity which can be addressed by introducing horizontal
stiffening fibers. An adequately stiff material with negative
Poisson's ratio is a simpler solution.
* * * * *